1. Field of the Invention
The present invention relates to a ceramic capacitor and a method of manufacturing the same.
2. Description of the Prior Art
In a conventional method of manufacturing a ceramic capacitor, a ceramic material having a high dielectric constant is shaped into a plate and sintered, and is provided with electrodes on both major surfaces. In such a ceramic capacitor, the thickness of the ceramic sintered body is reduced to a minimum in order to increase its capacitance and reduce its size. In a disc-type capacitor, which is a typical single-layer ceramic capacitor, for example, the thickness of the ceramic sintered body is 100 .mu.m at the minimum, with a rating of 50 V.
However, such a thin ceramic sintered body is low in mechanical strength. Thus, the ceramic capacitor may be broken not only at the stage of being a finished product also during the steps of manufacturing the ceramic sintered body and forming electrodes on both major surfaces of the ceramic sintered body.
In a method of manufacturing a laminated ceramic capacitor, on the other hand, a plurality of ceramic green sheets which are printed or coated with conductor metal powder paste for forming internal electrodes are stacked and laminated with each other under pressure, and then sintered to obtain a sintered body. Thereafter external electrodes are provided on such sintered body.
In such a laminated ceramic capacitor, the capacitance per unit volume is determined by the dielectric constant of the ceramic material, the total area of the internal electrodes and the thickness of each dielectric layer. In order to obtain a small capacitor having large capacitance, therefore, it is necessary to employ a ceramic material having a high dielectric constant and reduce the thickness of the dielectric layer provided between each adjacent pair of internal electrodes. Thus, the thickness of each ceramic green sheet is set at about 20 .mu.m, at the minimum.
The ceramic green sheets thinner than 20 .mu.m are not easy to make flaw-free.
In addition such thin ceramic green sheets are easily mechanically damaged through handling. It is very difficult to handle such thin sheets automatically.
Further, the thin ceramic green sheets are easily deformed by solvent contained in the internal electrode paste.
A Japanese Patent Application entitled "Ceramic Capacitor", which was filed in the name of the Assignee and laid open to the public on Feb. 10, 1987 under Japanese Patent Laying-Open Gazette No. 31108/1987, discloses a ceramic capacitor comprising a ceramic body having an insulation layer and semiconductive layers formed on both surfaces of the insulation layer and capacitance extracting electrodes provided on the surfaces of the semiconductive layers respectively. In this ceramic capacitor, a ferroelectric ceramic body is obtained by firing barium titanate, for example, and paste containing a strong-reducing type metal is applied to both major surfaces of the ceramic body to form capacitance extracting electrodes and then baked. This ceramic capacitor can be obtained by (1) firing ferroelectric material such as barium titanate, (2) applying electrode paste containing a reducer element on both surfaces of the ceramic body, and (3) applying heat treatment to form semiconductive layers in the vicinity of the surfaces of the ceramic body, and to form capacitance extracting electrodes on the surfaces of the ceramic body.
However, the technique disclosed in the aforementioned Gazette is not applicable to a laminated ceramic capacitor. In order to manufacture a laminated ceramic capacitor, the ceramic body must already contain paste for forming internal electrodes when the same is fired. And such paste must contain noble metals, such as gold, platinum, palladium, which can withstand an oxidizing atmosphere, since a ceramic material is generally fired in an oxidizing atmosphere. However, among such noble metals, none reduces ceramic dielectrics in the atmosphere.
U.S. Pat. No. 4,451,869 entitled "Laminated Ceramic Capacitor", which was issued on May 29, 1984 and granted to Sakabe et al., disclosed a laminated ceramic capacitor comprising a dielectric member of a non-reducing ceramic material, which can be fired in a reducing atmosphere. However, the capacitance obtained in such a capacitor employing a non-reducing ceramic material is lower than that of a capacitor which is fired in the aforementioned oxidizing atmosphere, i.e., a capacitor employing a reducing ceramic material.